In the case of homogeneous catalysis, a catalyst is located in the same phase or in the same aggregate state as the reactants, e.g., together with the reactants in a fluid or as a gas intermixed with reactant gases.
For some homogenously catalyzed synthesis reactions, catalysts are used that contain precious metal, such as platinum, palladium and rhodium, and ligands specially tailored for synthesis. The relatively high costs for such catalysts are caused both by the contained precious metals and the most difficulty synthesized ligands. The costs of the ligands can exceed the value of the precious metal.
During the reaction, the homogeneous catalyst is present dissolved in the reaction solution. After the reaction is complete, processing is required in order to separate and then reclaim the catalyst from the product and non-converted reactant and to be able to feed it to a new reaction cycle if applicable. The processing takes place for example through a thin film evaporation, which is relatively complicated and represents a thermal load on the catalyst.
The separation of the catalyst through a membrane process is being researched and tested as a new method. For this, solvent-stable membranes must be available that have a very high retention rate (>99%) at a high flow rate, in order to ensure an economical procedure.
All methods for processing homogeneous catalysts result in losses of precious metals and ligands (i.e., loss of catalyst), since a complete separation of the catalyst from the unreacted reactants and from the reaction product is not possible. The occurring catalyst losses must be compensated for the following reaction cycle through the addition of a new catalyst and represent a considerable cost factor in addition to the complicated separation.
Up until now, homogeneous catalysts are available dissolved in the reaction solution. After the reaction is complete, the catalyst (precious metal and ligand) must be separated from the reaction mixture (products and reactants). Technically, this occurs through thermal separation procedures, e.g. thin film vaporizer, multi-phase catalysis or adsorptive removal of the heavy metal catalysts (Königsberger et al. Org. Proc. Res. Dev. 2003, 7, 733).
One technique currently being researched is the separation of the catalysts with solvent-stable membranes (U. Kragl, C. Deisbach, “Membrane Reactors in Homogeneous catalysis”. in “Applied homogeneous catalysis with organometallic Compounds”, B. Cornils, W. A. Herrmann (eds.), 2. edition, 941-952, 2002, Wiley-VCH).
Complete reclamation of the catalyst is not achieved in either of the procedures. Moreover, the processing is an additional work step, which creates personnel and energy costs.